Chemically-strengthened glasses have found wide-spread application in touch panels and portable displays because of their excellent strength and damage resistance. These properties are particularly important when the glass acts as a cover glass for a device that is exposed to high levels of contact with surfaces. The damage resistance of the chemically strengthened glasses is a direct result of surface compression layers formed on the glass substrate via ion-exchange. The surface compression is balanced by a tensile region in the interior of the glass. Surface compressions (CS) greater than 750 MPa and compressive layer depths (DOL) greater than 40 microns are readily achieved in glasses, for example, Gorilla™ Glass (Corning Incorporated). By comparison, ordinary soda-lime glass has been able to reach only modest surface compression (“CS”) and depth-of-layer (“DOL”), which are typically<500 MPa<15 micron, respectively.
Recently the touch panel industry has been interested in putting the touch sensor directly on the chemically-strengthened cover glass instead of laminating a separate touch panel structure to the cover glass as is the current practice (see FIG. 1). The most economical manufacturing process would be to pattern multiple touch sensors onto a single large sheet of chemically strengthened glass, and then cut out the individual parts from the sheet having the touch sensors thereon. The magnitude of compressive stress and the elastic energy stored in the central tension region of the chemically strengthened glass, however, makes mechanical cutting of the substrates difficult. Hence, most of the current production processes involve cutting and finishing the non-ion-exchanged glass substrate to shape beforehand and ion exchanging afterwards. In this case, the touch sensor would have to be patterned separately onto each individual part (a “piece-part” process), which is not as economical as the “full sheet” process.
Methods for cutting tempered and chemically strengthened glass substrates have been disclosed in several patent and patent applications publications [for example, see, U.S. Pat. No. 4,468,534, US 2008-0128953, US 2010-0206008, US 2010-0210442, and JP 2008-007384], some of which are suitable for separating highly strengthened glass. It has been disclosed that setting limits on the stress profile can allow conventional cutting methods to be employed [for example, see US 2005-0221044, JP 2008-007369, JP 2004-352535, JP 2004-083378, GB 1222181 and WO 2008-108332]. However, the latter methods suffer by limiting the level of compression and/or depth of layer and, therefore, damage resistance that can be achieved in the glass. A serious drawback of both cutting techniques is the fact that the edge of the article after the separation process is not ion-exchanged and is therefore subject to damage and possible delayed failure (fatigue). Consequently, it is desirable to have a process which provides compression on the edge(s) of the parts after the separation process for both damage and fatigue resistance.
While high edge strength can be accomplished by acid etching the edge after the separation process using a coating for protecting the glass surface [for example, see commonly assigned U.S. patent application Ser. No. 12/862,096], there is still a problem. That problem is maintaining such strength. If the edge gets damaged post-acid treatment, the strength would be reduced. In other words, this process does not protect the glass from damage, for example, during use by a consumer. Consequently, it is desirable to have a process that provides compression on the edge of an article, after the separation of the article for a large glass sheet, with regard to both damage and fatigue resistance.
It is known that the films that comprise the touch sensor function are sensitive to high temperatures, and such film are usually limited to withstanding temperatures lower than 200° C.; that is, this is the maximum temperature to which such films should be subjected. This disclosure relates to locally ion-exchanging the glass edge(s) while maintaining the glass surface at temperatures<200° C., particularly when the glass has a touch sensor on a surface.